Thermoplastic polyolefin compositions are actively pursued as replacement materials for polyvinyl chloride based skin materials for the fabrication of many articles. In the automotive field, thermoplastic polyolefin compositions have been used for the fabrication of articles such as interior sheathing, including instrument panel skins, door panels, air bag covers and seat covers.
Many of the articles have surface appearances and designs having complex surface characteristics, such as contours and geometric technical grains, and may be produced in a slush molding process. However, the balance of material properties desired for a slush molding process is difficult to achieve with current thermoplastic polyolefin compositions. Current thermoplastic polyolefin compositions are often processed for prolonged time periods at extremely high temperatures to form a fused skin in a slush molding process. The material composition of such a typical thermoplastic polyolefin composition may degrade during processing which in turn may alter the material properties, such as the material strength and uniform fusion of the composition. As a result, articles produced using these thermoplastic polyolefin compositions may have unacceptable surface appearance and mechanical properties. To achieve suitability for slush molding without material property degradation, thermoplastic polyolefin compositions with a very low melt viscosity during the molding process are desired. Herein we refer to melt viscosity at any given temperature as that property measured at low shear rates, such as that defined by zero shear rate viscosity. The melt viscosity of the thermoplastic polyolefin compositions for use in slush molding should be, but are not limited to, in the range of 50 Pa·s to 250 Pa·s over the processing temperature range of 180° C. to 260° C. as measured at low shear rate such as that applied by parallel plate rheometer
Thus there is a need in the art for a thermoplastic polyolefin composition having a low melt viscosity at the molding temperature for use in slush molding. There is a further need for a thermoplastic composition having improved material properties, such as uniform melt fusion, during the slush molding process. There is still further need to convert the composition into a suitable powder (avg. particle size in the range of 75 μm to 400 μm) and/or micropellet form, (avg. particle size in the range of 300 μm to 900 μm), with a good mechanical flow during the slush molding process. There is a further need in the art for a process for preparing such a composition for use in manufacturing automotive and non-automotive articles with improved surface characteristics and appearance.